1. Field of the Invention
This invention generally relates to an electric lamp having a quartz glass envelope with a pinch seal and more particularly to a molybdenum foil coupling member used at the pinch seal.
2. Description of the Prior Art
Electric lamps having a quartz glass with a pinch seal in which a foil coupling member is incorporated as a current lead-through conductor are used widely in discharge lamps, in halogen lamps and so on.
The foil coupling member used in such lamps are usually made of pure molybdenum. In spite of the considerably different coefficients of thermal expansion of the quartz glass (approximately 5-6.times.10.sup.-7 per deg. C.) and of molybdenum (55.times.10.sup.-7 per deg. C.) lamps having vacuum-tight pinch seals are nevertheless obtained. This is due to the ductility of molybdenum, to the shape of the foil, the knife edge of the foil extending in the longitudinal direction of the pinch, and to the small thick thickness of the foil, which is about 20-40 microns.
In a halogen lamp, for example, the molybdenum foil coupling member is respectively connected to an internal current conductor, in most cases consisting of tungsten, which has a much higher melting point than molybdenum and of molybdenum.
Lamps of the type described above are known from U.S. Pat. No. 4,254,300. In such lamps, the molybdenum foil contains yttrium oxide (Y.sub.2 O.sub.3) in a quantity of from 1/4 to 1% of the molybdenum weight to avoid fracture at a welded joint between the molybdenum foil coupling member and the internal current conductor.
A more serious problem than the aforementioned fracture, is cracking or tearing off of the molybdenum foil itself which mainly occurs during the sealing step, i.e., after making the weld between the molybdenum foil coupling member and internal and external current conductors when the sealing step proceeds. At the sealing step between the molybdenum foil coupling member and the quartz glass envelope, the end of the quartz glass envelope is heated with sealing burners above its melting point (more than 1500.degree. C.). The pure molybdenum foil coupling member is simultaneously heated to a very high temperature by the sealing burners. After that, the end of the envelope is pinched with pinching jigs, and a pinch seal is formed.
Although molybdenum foil coupling members show high tensile strength, i.e., 70-90 kg/mm.sup.2 in the normal temperature range, the molybdenum foil coupling member occasionally becomes torn or cracked owing to high temperatures during the sealing step.
The cracking or tearing of the molybdenum foil coupling member is a cause of short lamp life, bad vacuum seal and results in unusable articles of inferior quality.